1. Field of the Invention
The present invention relates to a method and apparatus for press-molding optical glass elements or the like such as a lens for a camera and spectacles.
2. Description of the Related Art
The apparatus of this type is known which press-molding a glass blank between a pair of mutually opposite molds, upper and lower, through the heating of the molds and glass blank by means of a heating means located around these molds.
The heating means used for such apparatus includes means using an infrared lamp or a high-frequency coil.
From various studies on the application of heat by infrared lamp, the inventors have found that, if, depending upon the shape and size of a molding optical glass element, etc., and those of the molds, the relative position of the infrared lamps to the molds varies, a different temperature gradient occurs on the upper and lower molds upon the heating of these molds to a predetermined molding temperature and it takes a longer time to control the outputs of the individual infrared lamps so that a uniform temperature distribution can be obtained.
It has also been found that, even when the relative position of the infrared lamps to the molds varies after the upper and lower molds have been stabilized to a predetermined molding temperature, a temperature difference on the upper and lower molds is relatively slight and that the upper and lower molds can be controlled to a predetermined temperature by the output control of the individual infrared lamps.
In the apparatus for press-controlling a glass element through the heating of the glass element and molds by a high-frequency coil, the pair of molds, upper and lower, are controlled to a predetermined temperature normally by the detection of one of the upper and lower molding temperatures and output control of the high-frequency coil.
If two or more high-frequency coils are arranged near each other, no proper heating can be achieved due to their mutual interference and, for this reason, the upper and lower molds are heated with the use of one high-frequency coil. Since one high-frequency coil is employed, temperature control is carried out based on one or the other molding temperature and a temperature balance on the upper and lower molds is achieved by the relative positional relation of the high-frequency coil to the upper and lower molds in the longitudinal direction of the high-frequency coil, fine/coarse pitch of the high-frequency coil, and so on.
In the case where, as shown in FIG. 8A, a glass blank is press-molded by, for example, moving a lower mold 52 alone with an upper mold 51 fixedly held in place relative to a high-frequency coil 50, when the lower mold 52 is moved from a position L.sub.1 near the lower end of the high-frequency coil 50 to a near-middle position L.sub.4 thereof, the temperature of the lower mold 52 rises as indicated by a solid line T.sub.2 in FIG. 8B even if a high-frequency output is constant.
This temperature rise indicates that the heating by the high-frequency coil 50 is more higher at the lower end side than at the middle side.
In the case where, with the lower mold 52 in the L.sub.1 position, both the molds 51 and 52 are set at a temperature Tm in FIG. 8B, then with the lower mold 52 in the closed state corresponding to the L.sub.4 position the lower mold 52 goes higher in temperature than the upper mold 51 since the upper mold 51 experiences no temperature variation as indicated by T.sub.1a in FIG. 8B.
For this reason it has been usual practice to initially set the upper mold 51 to a set temperature Tn higher by a predetermined amount as indicated by a dash-dot line in FIG. 8B and to secure a desired temperature difference or a uniform temperature between these molds when the lower mold 52 reaches the position L.sub.4.
When, in the apparatus shown in FIG. 8A, the high-frequency coil 50 is output-controlled so as to have the temperature of the lower mold 52 maintained at a constant level as indicated by a solid line T.sub.2 ' in FIG. 8C, the temperature of the upper mold 51 is gradually lowered with a movement of the lower mold 52 from the L.sub.1 position to the L.sub.4 position as indicated by broken lines T.sub.1a ' and T.sub.1b ' in FIG. 8C and a relative temperature difference between the upper and lower molds 51 and 52 is substantially as shown in FIG. 8C.
It has also been the usual practice to initially heat the temperature of the upper mold 51 by a predetermined amount to a set level as indicated by the broken line T.sub.1b ' in the case where the temperature of the lower mold 52 is used as a reference level as shown in FIG. 8C.
The temperature variation as indicated by the solid line T.sub.2 can be suppressed by using a high-frequency coil of adequate length or a high-frequency coil of fine pitches. However, increasing the length of the high-frequency coil 50 involves various drawbacks and the coil pitch is very delicate to adjust. Further it takes a greater time to make such an adjustment as set out above.